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3D simulation of sputter etching with the Monte-Carlo approach

3D Simulation des Zerstäubungsansätzen mittels der Monte-Carlo Methode
: Kunder, D.

Volltext ()

Erlangen-Nuernberg, 2011, XIV, 128 S.
Erlangen-Nürnberg, Univ., Diss., 2011
URN: urn:nbn:de:bvb:29-opus-22987
Dissertation, Elektronische Publikation
Fraunhofer IISB ()
sputter etching; simulation; Monte-Carlo simulation; focused ion beam

Sputtering as physical process is frequently used for structuring in technology. A typical example is focused ion beam (FIB) milling with which submicron-sized structures can be produced out of an arbitrary material without the requirement of lithography. As a second prominent example, sputtering is used to sharpen the tip of field emitter of field emitter arrays. For all these processes, simulation of the topography changes due to sputtering can be very useful to understand and optimize them. A key part of this work is the integration of the Monte-Carlo ion implantation simulator MC SIM into the 3-D topography simulator ANETCH. This way, ANETCH is now able to simulate topography changes due to sputtering for a wide range of ion/target combinations without a-priori knowledge about the res pective yield from experiments. Although the models used in Monte-Carlo programs are usually accurate enough for simulating ion implantation profiles, this is not necessarily true for sputtering. In particular, to improve the accuracy of the simulations, a modification of the electronic stopping model in MC SIM is suggested. Another limitation of Monte-Carlo programs is that they consider only planar surfaces. In the implementation performed in this work, the full surface topography provided by ANETCH was taken into account to calculate sputtering. This is particularly important at corners where sputtering may occur even when the respective surface segment is not exposed to the ion beam. Using the implementation of MC SIM into ANETCH, characteristic features of sputtering were investigat ed. The most important ones occurring particularly during FIB processing are the formation of sloped side walls and an increase of the etch rate at the bottom of trenches close to the side walls known as microtrenching. The main influencing factors determining the slopes of the side walls like redeposition, the spatial distribution of the ion fluence, and the angular dependence of the sputtering yield are discussed in detail. Microtrenching was found to depend particularly on the reflection of the ions at the side walls. A comparison with the often found assumption of a specular reflection of ions at sloped side walls indicates that the ions are in reality reflected further away from the side walls which leads to a less pronounced but spatially more extended microtrenching than predicted b y the simple assumption of specular reflection. Important for the sharpening of tips is also the side-wall propagation at a steep step due to a uniform ion irradiation. Simulations with ANETCH were able to reproduce this phenomenon and an analytical model was developed that explains the slope of the side wall. To validate the implementation of the integration of MC SIM into ANETCH, the topographies of trenches fabricated by FIB milling with different currents were compared to the respective simulations. Good agreement was found and the experiments as well as the simulations showed that microtrenching is reduced and eventually vanishes for higher beam currents because of the more pronounced spreading of the beam. Finally, to validate the dependence of the sputtering yield on the angle of i ncidence, dedicated FIB experiments were designed first by simulations. A comparison of the respective experimental results to simulations confirmed the suitability of the model particularly for FIB processing.